Like someone else mentioned, "faster" depends on what you are doing. I can't tell a difference between my 2,8 ghz iMac and my 1,67 ghz pb G4 in iTunes, Safari, Mail, iCal,... even opening vector graphics in Illustrator CS3 or not too complex documents in InDesign CS3 is about the same speed.
The difference becomes noticeable when Photoshop documents go beyond 150 - 200 mb, which they often do. 😉
Or when rendering a complex image in Cinema 4D, the iMac will do it in 20 seconds where the pb needs 3 minutes.
And of course when converting homemade movies, where a 6 hours job on the pb lasts 30 minutes on the iMac.